In the present study we used Western blot techniques to investigate the expression of aFGF and bFGF in the rat embryo, and we used immunohistochemical techniques to define the cellular and tissue distribution of aFGF and bFGF in embryos at different stages of development (from 11 to 20 days). Forms of 26 to 29 kD for aFGF and bFGF were identified both in crude tissue extracts and heparin-bound material, and showed mitogenic activity for 3T3 fibroblasts. Thus, they appear to be the main natural forms of aFGF and bFGF in the rat embryo. Immunoreactive aFGF and bFGF were identified in the cytoplasm of cells of neuroectodermal and mesodermal origin but not in endoderm-derived cells, and they were ubiquitous in the extracellular matrix. The distribution of aFGF and bFGF also showed changes during development that were associated with the process of cellular and tissue differentiation. For example, intensity and extent of immuno- reactivity for both peptides progressively increased in the middle layer of the spinal cord with increasing differentiation of the neural cells. The immunostaining patterns for aFGF and bFGF were virtually superimposable for each organ and at each stage. In conclusion, the nonuniform but specific tissue distribution of aFGF and bFGF and the changes in their distribution during embryonic development, suggest that these peptides have different regulatory effects in different cells and tissues during organogenesis. Their coexpression implies some form of functional interaction.